Wireless communication apparatus

ABSTRACT

A wireless communication apparatus includes a transmitter that transmits a first frame and a second frame with multiplexing and a receiver that receives a third frame sent in response to the first frame and a forth frame sent in response to the second frame, wherein a first receipt completion timing at which the third frame is fully received by the receiver and a second receipt completion timing at which the forth frame is fully received by the receiver are synchronized by controlling at least one of first content of the first frame, a first transmission method by which the first frame is sent, second content of the second frame and a second transmission method by which the second frame is sent.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation of U.S. patent application Ser. No.15/399,223 filed Jan. 5, 2017, which is a Continuation of U.S. patentapplication Ser. No. 14/854,202 filed Sep. 15, 2015, which is aContinuation of U.S. patent application Ser. No. 14/162,887 filed Jan.24, 2014, which is a Continuation of U.S. patent application Ser. No.12/402,699 filed Mar. 12, 2009, the entire contents of which areincorporated herein by reference.

The present disclosure relates to the subject matters contained inJapanese Patent Application No. 2008-075222 filed on Mar. 24, 2008,which are incorporated herein by reference in its entirety.

FIELD

The present invention relates to a wireless communication apparatus anda method and program for controlling the wireless communicationapparatus.

BACKGROUND

CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is usedin IEEE802.11 which is a typical standard for communication used in awireless LAN.

In CSMA/CA, each wireless communication apparatus transmits a data frameafter performing carrier sensing to confirm that a channel to be used isbeing unused (idle) for more than a predetermined time period.

When a wireless communication apparatus receives the data frameproperly, the wireless communication apparatus returns a receiptconfirmation frame after a predetermined period (an SIFS (Short InterFrame Space)) to notify the data frame sender of the normal receipt ofthe data frame. The receipt confirmation frame may be an ACK frame, aBlock ACK frame, or a CTS frame. The wireless communication apparatus onthe receiver side answers with the receipt confirmation frame whichvaries according to the type of the received data frame.

SDM (Space Division Multiplexing) is known as one of wirelesscommunication systems. In SDM, a wireless communication apparatus on thesender side forms directional radio waves by using an adaptive arrayantenna or the like and transmits (spatially multiplexes) differentradio waves to a plurality of wireless communication apparatusessimultaneously and with one and the same frequency. That is, in SDM, awireless communication apparatus on the sender side (e.g. an accesspoint device) can transmit different data frames to a plurality ofwireless terminals respectively, simultaneously and with one and thesame frequency.

When SDM is used in an IEEE802.11 standard-based wireless LAN system,there is however a possibility of occurrence of a situation where theaccess point device may receive a receipt confirmation frame fromanother wireless terminal while the access point device is transmittinga data frame to one wireless terminal. This situation is referred to asa “collision.”

The situation how the collision occurs will be described below.

First, the access point device starts transmission of different dataframes to a plurality of wireless terminals simultaneously and with oneand the same frequency by SDM. When the size of a data frame to betransmitted to one wireless terminal STA1 is large compared with thesizes of data frames to be transmitted to other wireless terminals, theaccess point device may terminate transmission of the data frames to theother wireless terminals than the wireless terminal STA1 while theaccess point device transmits the data frame to the wireless terminalSTA1. Because the other wireless terminals than the wireless terminalSTA1 return receipt confirmation frames, there is a possibility ofoccurrence of a situation (collision) that the access point device mayreceive the receipt confirmation frames from the other wirelessterminals than the wireless terminal STA1 though the access point deviceis transmitting the data frame to the ratio terminal STA1. As describedabove, when SDM is used in an IEEE802.11 standard-based wireless LANsystem, there is a problem that the access point device cannot receiveany receipt confirmation frame.

As a method to solve this problem, there has been disclosed a techniquein which the return start timings of receipt confirmation frames from aplurality of wireless terminals are synchronized when different dataframes are transmitted to the plurality of wireless terminalssimultaneously and with one and the same frequency by SDM. An example ofsuch technique is disclosed in the following document (1).

US 2005/0147023 A1 (1)

In the technique disclosed in the document (1), occurrence of collisioncan be prevented even when SDM is employed in a wireless LAN systemcompliant with the IEEE802.11 standard.

As shown in FIG. 9, the IEEE802.11e standard which is an enhancedIEEE802.11 MAC layer protocol allows data frames to be transmittedcontinuously on an exclusively used channel during a period called TXOP(Transmission Opportunity) after the authorization of data frametransmission is acquired once.

During the TXOP period, any number of data frames is allowed to betransmitted continuously at intervals of SIFS, as described in thefollowing document (2).

“Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)specifications, Medium Access Control (MAC) Quality of Service (QoS)Enhancements”, IEEE Std. 802.11e-2005 (2)

In the technique described in the document (2), because it isunnecessary to acquire the authorization of data frame transmissionagain whenever a data frame is transmitted, the overhead (AIFS(Arbitration Short Inter Frame Space) and backoff period) required foracquiring the authorization of data frame transmission can be reduced toachieve improvement in throughput and system capacity.

As described above, in the technique disclosed in the document (1), evenwhen SDM is employed in the IEEE802.11 standard, occurrence of collisioncan be suppressed by synchronizing the timings of receipt confirmationframes returned from the respective wireless terminals.

However, when SDM is employed in the technique disclosed in the document(2), that is, in the technique in which data frames are transmittedcontinuously during the period of TXOP defined in IEEE802.11e, thefollowing problem occurs.

Assume now that an access point device transmits data framescontinuously to a plurality of wireless terminals simultaneously andwith one and the same frequency during the TXOP period. The access pointdevice transmits next data frames without carrier sensing again when anSIFS has passed after reception of receipt confirmation frames asreplies to the transmitted data frames from the plurality of wirelessterminals.

The receipt confirmation frames are various in terms of type and size.The transfer rate in transmission of a receipt confirmation frame to theaccess point device varies according to each wireless terminal. For thisreason, even when the timings of returning receipt confirmation framesfrom the plurality of wireless terminals are synchronized by thetechnique disclosed in the document (1), the time required forcompletion of the access point device's reception of a receiptconfirmation frame returned from each wireless terminal (the timerequired for delivery of a frame (hereinafter referred to as frame timelength)) varies.

Although the access point device transmits a next data frame when theSIFS has passed after reception of a receipt conformation frame, thetime point of transmission start of the next data frame varies becausethe time required for completion of reception of the receiptconformation frame varies.

For this reason, when the time length difference between a receiptconfirmation frame from the wireless terminal STA1 and a receiptconfirmation frame from the wireless terminal STA2 exceeds the SIFS asshown in FIG. 10A, transmission of a next data frame to the wirelessterminal STA1 is started though reception of the receipt confirmationframe the wireless terminal STA2 is not completed. There is a problemthat collision occurs.

On the other hand, if transmission of a next data frame is intended tobe started when a predetermined time (SIFS) has passed after completionof reception of the latest receipt confirmation frame as shown in FIG.10B, the channel seems free for the SIFS to the wireless terminal (thewireless terminal STA2 in FIG. 10B) returning the receipt confirmationframe latest in terms of reception in the access point device but thechannel seems free for a period longer than the SIFS to the otherwireless terminal (the wireless terminal STA1 in FIG. 10B) than thewireless terminal returning the receipt confirmation frame latest interms of reception in the access point device. For this reason, there isa possibility that the other wireless terminal than the wirelessterminal returning the receipt confirmation frame latest in terms ofreception in the access point device will acquire the authorization oftransmission even when the access point device is continuouslytransmitting data frames.

In such a case, data frames transmitted continuously by the access pointdevice collide with a data frame transmitted by the wireless terminalwhich acquires the authorization of transmission because the channel isfree for a period longer than the SIFS. There is hence a problem thatthe access point device cannot transmit data frames continuously atintervals of the SIFS during the TXOP period.

As described above, in the IEEE802.11-based wireless LAN system, whenthe access point device transmits spatially multiplexed data frames to aplurality of wireless terminals and transmits such data framescontinuously at intervals of the SIFS during the TXOP period, there is aproblem that collision will occur because the transmission timing of adata frame to one wireless terminal overlaps with the reception timingof a receipt confirmation frame from any other wireless terminal.

For this reason, there is a problem that MAC efficiency cannot beimproved because the concept of TXOP (the technique for transmittingdata frames continuously at intervals of the SIFS during the TXOPperiod) cannot be applied to the case where SDM is used in theIEEE802.11-based wireless LAN system for spatially multiplexing andtransmitting data frames.

SUMMARY

According to a first aspect of the present invention, there is provideda wireless communication apparatus including: a communication moduleconfigured to wirelessly communicate with a first counterpartcommunication apparatus and a second counterpart communication apparatusthrough a wireless channel during a period in which the wireless channelis allowed to be exclusively used; and a controller configured tocontrol the communication module, wherein the communication modulemultiplexes a first data frame and a second data frame and transmits thefirst data frame to the first counterpart communication apparatus with afirst transmission method and the second data frame to the secondcounterpart communication apparatus with a second transmission method,wherein the communication module completes receiving a first receiptconfirmation frame being sent from the first counterpart communicationapparatus with a first returning method at a first receipt completiontiming and a second receipt confirmation frame from the second counterapparatus with a second returning method at a second receipt completiontiming, wherein the first receipt confirmation frame and the firstreturning method are determined based on the first data frame and thefirst transmission method, wherein the second receipt confirmation frameand the second returning method are determined based on the second dataframe and the second transmission method, wherein the controllercontrols the communication module to transmit the first data frame inthe first transmission method and the second data frame in the secondtransmission method so as to synchronize the first receipt completiontiming and the second receipt completion timing, and wherein thecommunication module transmits a next first data frame and a next seconddata frame after elapse of a given time from the first receiptcompletion timing and the second receipt completion timing.

According to a second aspect of the present invention, there is provideda wireless communication apparatus including: a communication moduleconfigured to wirelessly communicate with a first counterpartcommunication apparatus and a second counterpart communication apparatusthrough a wireless channel during a period in which the wireless channelis allowed to be exclusively used; and a generation module configured togenerate notification information for controlling the first counterpartcommunication apparatus and the second counterpart communicationapparatus to synchronize a first receipt completion timing, at which thecommunication module completes receiving a first receipt confirmationframe being sent from the first counterpart communication apparatus, anda second receipt completion timing, at which the communication modulecompletes receiving a second receipt confirmation frame being sent fromthe second counterpart communication apparatus, wherein thecommunication module multiplexes a first data frame and a second dataframe and transmits the first data frame to the first counterpartcommunication apparatus and the second data frame to the secondcounterpart communication apparatus while transmitting the notificationinformation to the first counterpart communication apparatus and thesecond counterpart communication apparatus, wherein the communicationmodule transmits a next first data frame and a next second data frameafter elapse of a given time from the first receipt completion timingand the second receipt completion timing.

According to a third aspect of the present invention, there is provideda wireless communication apparatus including: a communication moduleconfigured to wirelessly communicate with a first counterpartcommunication apparatus and a second counterpart communication apparatusthrough a wireless channel during a period in which the wireless channelis allowed to be exclusively used, wherein the communication modulemultiplexes a first data frame and a second data frame and transmits thefirst data frame to the first counterpart communication apparatus andthe second data frame to the second counterpart communication apparatus,wherein the communication module completes receiving a first receiptconfirmation frame being sent from the first counterpart communicationapparatus at a first receipt completion timing and a second receiptconfirmation frame from the second counter apparatus at a second receiptcompletion timing, the second receipt completion timing being insynchronization with the first receipt completion timing, and whereinthe communication module transmits a next first data frame and a nextsecond data frame after elapse of a given time from the first receiptcompletion timing and the second receipt completion timing.

According to a fourth aspect of the present invention, there is provideda wireless communication apparatus including: a communication moduleconfigured to wirelessly communicate with a first counterpartcommunication apparatus and a second counterpart communication apparatusthrough a wireless channel during a period in which the wireless channelis allowed to be exclusively used, wherein the communication modulemultiplexes a first data frame and a second data frame and transmits thefirst data frame to the first counterpart communication apparatus andthe second data frame to the second counterpart communication apparatus,wherein the communication module completes receiving a first receiptconfirmation frame being sent from the first counterpart communicationapparatus at a first receipt completion timing and a second receiptconfirmation frame from the second counter apparatus at a second receiptcompletion timing, and wherein the communication module transmits a nextfirst data frame and a next second data frame after elapse of a giventime from the first receipt completion timing and the second receiptcompletion timing when the first receipt completion timing and thesecond receipt completion timing are in synchronization with each other.

According to a fifth aspect of the present invention, there is provideda method for controlling a wireless communication apparatus configuredto wirelessly communicate with a first counterpart communicationapparatus and a second counterpart communication apparatus through awireless channel during a period in which the wireless channel isallowed to be exclusively used, the method including: multiplexing afirst data frame and a second data frame; transmitting the first dataframe to the first counterpart communication apparatus and the seconddata frame to the second counterpart communication apparatus; completereceiving a first receipt confirmation frame being sent from the firstcounterpart communication apparatus at a first receipt completion timingand a second receipt confirmation frame from the second counterapparatus at a second receipt completion timing, the second receiptcompletion timing being in synchronization with the first receiptcompletion timing, and transmitting a next first data frame and a nextsecond data frame after elapse of a given time from the first receiptcompletion timing and the second receipt completion timing.

According to a sixth aspect of the present invention, there is provideda computer-readable storage medium containing a program for causing awireless communication apparatus to operate in accordance with asequence of procedure, the wireless communication apparatus beingconfigured to wirelessly communicate with a first counterpartcommunication apparatus and a second counterpart communication apparatusthrough a wireless channel during a period in which the wireless channelis allowed to be exclusively used, the procedure including: multiplexinga first data frame and a second data frame; transmitting the first dataframe to the first counterpart communication apparatus and the seconddata frame to the second counterpart communication apparatus; completereceiving a first receipt confirmation frame being sent from the firstcounterpart communication apparatus at a first receipt completion timingand a second receipt confirmation frame from the second counterapparatus at a second receipt completion timing, the second receiptcompletion timing being in synchronization with the first receiptcompletion timing; and transmitting a next first data frame and a nextsecond data frame after elapse of a given time from the first receiptcompletion timing and the second receipt completion timing.

BRIEF DESCRIPTION OF THE DRAWINGS

A general configuration that implements the various feature of theinvention will be described with reference to the drawings. The drawingsand the associated descriptions are provided to illustrate embodimentsof the invention and not to limit the scope of the invention.

FIG. 1 is a diagram showing a wireless system according to a firstembodiment of the invention.

FIG. 2 is a block diagram showing the configuration of an access pointdevice according to the first embodiment of the invention.

FIG. 3 is a block diagram showing the configuration of a controlleraccording to the first embodiment of the invention.

FIGS. 4A and 4B are views showing frames exchanged between the accesspoint device and a plurality of wireless terminals.

FIG. 5 is a block diagram showing the configuration of a returncompletion control module according to the first embodiment of theinvention.

FIGS. 6A and 6B are views frames exchanged between the access pointdevice and the plurality of wireless terminals.

FIG. 7 is a block diagram showing an example of configuration of theaccess point device according to a second embodiment of the invention.

FIG. 8 is a block diagram showing another example of configuration ofthe access point device according to a second embodiment of theinvention.

FIG. 9 is a view showing the concept of TXOP in the IEEE802.11estandard.

FIGS. 10A and 10B are views showing frames exchanged between the accesspoint device and the plurality of wireless terminals.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a diagram showing a wireless system according to a firstembodiment of the invention. The wireless system is compliant with theIEEE802.11e (including IEEE802.11a, IEEE802.11b and IEEE802.11n)standard. SDM (Spatial Division Multiplexing) is used in the wirelesssystem.

The wireless system according to the first embodiment includes an accesspoint device AP, and wireless terminals STA1 to STA3. Wirelesscommunication between the access point device AP and each of thewireless terminals STA1 to STA3 is performed by SDM. The access pointdevice AP transmits data frames continuously during a TXOP period asdefined in IEEE802.11e.

The access point device AP creates radio waves with directivity(hereinafter referred to as directional beams) by using an adaptivearray antenna. The access point device AP creates the directional beamsso that the directional beams to be transmitted to the wirelessterminals STA1 to STA3 respectively do not interfere with one another.The access point device AP transmits the created directional beamsspatially multiplexed with data frames to the wireless terminals STA1 toSTA3 respectively, simultaneously and on one and the same frequencychannel (hereinafter referred to as SDM-transmission).

The number of directional beams created by the access point device AP,that is, the number of wireless terminals with which the access pointdevice AP communicates simultaneously by radio by SDM may be any integernot smaller than 2.

FIG. 2 is a block diagram showing the configuration of the access pointdevice AP according to the first embodiment.

The access point device AP has an antenna module 10, a communicationmodule 20, a MAC processing module 30, a controller 40, and amultiplexing module 50.

The antenna module 10 includes a plurality of antenna elements so thatdirectional beams can be created.

The communication module 20 performs reception processing such asdemodulation to a signal received from another wireless terminal throughthe antenna module 10. The communication module 20 performs transmissionprocessing such as modulation to a transmission signal to be transmittedto another wireless terminal.

The MAC processing module 30 performs processing concerned with a MAClayer in accordance with the IEEE802.11 (including IEEE802.11e) standardMAC protocol.

The controller 40 performs data frame control and transmission methodcontrol for data frames to be transmitted by SDM.

The multiplexing module 50 performs processing for data frames to betransmitted to a plurality of wireless terminals simultaneously and onone and the same frequency channel, for example, processing such asweight matrix calculation for creation of directional beams (hereinafterreferred to as multiplexing).

The communication module 20 performs transmission processing forsimultaneous transmission of data frames to a plurality of wirelessterminals and performs reception processing for simultaneous receptionof data from a plurality of wireless terminals by SDM. It is thereforepreferable that the transmission processing and reception processing inthe communication module 20 are performed as parallel processing.

Although the access point device AP shown in FIG. 2 has been describedon the assumption that the communication module 20 performs transmissionprocessing after the multiplexing module 50 performs processing for dataframes to be SDM-transmitted, the access point device AP may beconfigured so conversely that the multiplexing module 50 performsprocessing after the communication module 20 performs transmissionprocessing.

An operation when the access point device AP SDM-transmits data framesto the wireless terminals STA1 to STA3 will be described below.

First, data frames each having a transmission request in processingconcerned with an upper layer are stored in a built-in transmissionbuffer (not shown) included in the MAC processing module 30.

Then, the access point device AP acquires the authorization oftransmission by carrier sensing in accordance with the IEEE802.11standard MAC protocol.

Then, the controller 40 takes data frames to be transmitted to thewireless terminals STA1 to STA3 out of the built-in transmission bufferof the MAC processing module 30 and determines a transmission method foreach data frame.

Then, the multiplexing module 50 performs multiplexing for the dataframes under the control of the controller 40.

Then, the communication module 20 performs transmission processing forthe multiplexed data frames.

Then, directional beams from the antenna module 10 are transmitted tothe wireless terminals STA1 to STA3 respectively. In this manner, theaccess point device AP SDM-transmits data frames to the wirelessterminals STA1 to STA3.

Then, the access point device AP confirms receipt confirmation framesreturned from the wireless terminals STA1 to STA3 when an SIFS haspassed after transmission of the data frames.

When receipt confirmation frames are returned from the wirelessterminals STA1 to STA3 respectively, that is, when SDM-transmission ofdata frames to the wireless terminals STA1 to STA3 ends in success, theaccess point device AP performs SDM-transmission of data frames to thewireless terminals STA1 to STA3 continuously without carrier sensingwhen an SIFS has passed after completion of reception of the receiptconfirmation frames.

Each of the data frames which are continuously transmitted by the accesspoint device AP when the SIFS has passed after completion of receptionof the receipt confirmation frames may be a data frame transmitted toany one of the wireless terminals STA1 to STA3 or may be datatransmitted to another wireless terminal.

Unless a TXOP period has passed after acquisition of the transmissionauthorization or unless all data frames stored in the transmissionbuffer have been transmitted completely, the access point device APtransmits data frames continuously when the SIFS has passed aftercompletion of reception of the receipt confirmation frames.

When receipt confirmation frames are not returned from all or part ofthe wireless terminals STA1 to STA3, that is, when SDM-transmission ofdata frames to the wireless terminals STA1 to STA3 ends in failure, theaccess point device AP transmits new data frames to wireless terminalsreturning the receipt confirmation frames while re-transmitting dataframes to wireless terminals not returning the receipt confirmationframes.

The access point device AP gives priority to re-transmission of dataframes over transmission of new data frames. When, for example, theaccess point device AP receives receipt confirmation frames from onlythe wireless terminals STA2 and STA3 though the access point device APhas SDM-transmitted data frames to the wireless terminals STA1 to STA3,the access point device AP performs re-transmission of a data frame tothe wireless terminal STA1 and transmission of new data frames to thewireless terminals STA2 and STA3, simultaneously and on one and the samefrequency channel (by SDM).

Alternatively, when receipt confirmation frames are not returned fromall or part of the wireless terminals STA1 to STA3, that is, whenSDM-transmission of data frames to the wireless terminals STA1 to STA3ends in failure, the access point device AP may re-transmit data framesto only wireless terminals not returning the receipt confirmationframes.

When, for example, the access point device AP receives a receiptconfirmation frame from only the wireless terminal STA2 though theaccess point device AP has SDM-transmitted data frames to the wirelessterminals STA1 STA3, the access point device AP re-transmits data framesto the wireless terminals STA1 and STA3 simultaneously and on one andthe same frequency channel (by SDM).

FIG. 3 is a block diagram showing the configuration of the controller40. An operation of the controller 40 when the access point device APSDM-transmits data frames to the wireless terminals STA1 to STA3 will bedescribed below in connection with the configuration of the controller40.

The controller 40 has a return start control module 41, and a returncompletion control module 42.

The return start control module 41 controls SDM-transmitted data framesand a transmission method thereof when the data frames areSDM-transmitted to the wireless terminals STA1 to STA3 so that thewireless terminals STA1 to STA3 on the receiver side will startreturning of receipt confirmation frames at synchronized timing(hereinafter referred to as return start timing).

Any method may be used as the control method performed by the returnstart control module 41 if the return start timings of the wirelessterminals STA1 to STA3 on the receiver side of SDM-transmission can besynchronized with one another. The control method performed by thereturn start control module 41 will be described by way of example asfollows.

First Example of Control Method

The return start control module 41 equalizes the frame time lengths ofdata frames to be SDM-transmitted to the wireless terminals STA1 toSTA3.

For example, the return start control module 41 equalizes the framelengths of data frames to be SDM-transmitted to the wireless terminalsSTA1 to STA3 and equalizes the transfer rates in SDM-transmission ofdata frames to the wireless terminals STA1 to STA3 to thereby equalizethe frame time lengths of the data frames to be SDM-transmitted to thewireless terminals STA1 to STA3.

Alternatively, for example, the return start control module 41 equalizesthe ratio of the frame length of a data frame SDM-transmitted to awireless terminal to the transfer rate in SDM transmission of the dataframe to the wireless terminal, among the wireless terminals STA1 toSTA3 to thereby equalize the frame time lengths of the data frames to beSDM-transmitted to the wireless terminals STA1 to STA3.

That is, for example, the return start control module 41 can performcontrol in such a manner that the frame length of a data frame to beSDM-transmitted to the wireless terminal STA1 is set to be twice aslarge as the frame length of a data frame to be SDM-transmitted to thewireless terminal STA2 while the transfer rate in SDM-transmission tothe wireless terminal STA1 is set to be twice as high as the transferrate in SDM-transmission to the wireless terminal STA2.

In this manner, the return start control module 41 synchronizes dataframes SDM-transmitted from the access point device AP to the wirelessterminals STA1 to STA3 so that transmission of the data frames can becompleted at synchronized timing (hereinafter referred to astransmission completion timing). Each of the wireless terminals STA1 toSTA3 starts returning of a receipt confirmation frame when an SIFS haspassed after the transmission completion timing. Accordingly, the returnstart timings of the wireless terminals STA1 to STA3 on the receiverside of SDM-transmission are synchronized with one another.

Second Example of Control Method

When the frame time lengths of data frames to be SDM-transmitted to thewireless terminals STA1 to STA3 are different, the return start controlmodule 41 delays the timing of start of transmission (hereinafterreferred to as transmission start timing) of any other data frame than adata frame largest in frame time length for a difference between theframe time lengths of the data frames so that the transmissioncompletion timing of the data frame largest in frame time length can besynchronized with the transmission completion timing of any other dataframe.

In this manner, like the first example, the transmission completiontimings of data frames are synchronized with one another, so that thereturn start timings of receipt confirmation frames are synchronizedwith one another.

The aforementioned synchronization of the transmission completiontimings of data frames and the return start timings of receiptconfirmation frames need not mean complete synchronization so that thedifference between the return start timings of receipt confirmationframes from a plurality of wireless terminals can be allowed to be notlarger than the SIFS (16 microseconds).

However, for reception of receipt confirmation frames from three or morewireless terminals, it is necessary that the difference between thereturn completion timing of a receipt confirmation frame earliest incompletion of reception thereof and the return completion timing of areceipt conformation frame latest in completion of reception thereof isnot larger than the SIFS (16 microseconds).

Although description has been made on the case where the differencebetween the return start timings of receipt confirmation frames from aplurality of wireless terminals can be allowed to be not larger than theSIFS (16 microseconds), the difference may be set, for example, to benot larger than the time length of a preamble of each data frame.

The return completion control module 42 controls SDM-transmitted dataframes and a transmission method thereof when data frames areSDM-transmitted to the wireless terminals STA1 to STA3 so that returningof receipt confirmation frames from the wireless terminals STA1 to STA3on the receiver side can be completed at synchronized timing(hereinafter referred to as return completion timing).

Any method may be used as the control method performed by the returncompletion control module 42 if the return completion timings of receiptconfirmation frames from the wireless terminals STA1 to STA3 on thereceiver side of SDM-transmission can be synchronized with one another.The control method performed by the return completion control module 42will be described later in detail.

As described above, in the access point device AP, the return startcontrol module 41 synchronizes the return start timings of receiptconfirmation frames returned from the wireless terminals STA1 to STA3 onthe receiver side of SDM-transmission of data frames, and the returncompletion control module 42 synchronizes the return completion timingsof the receipt confirmation frames returned from the wireless terminalsSTA1 to STA3 on the receiver side of SDM-transmission of the dataframes.

In this manner, even when the access point device AP SDM-transmits dataframes to the wireless terminals STA1 to STA3 so that the data framescan be transmitted continuously without carrier sensing in a TXOPperiod, the transmission of the data frames by the access point deviceAP can be prevented from overlapping with the reception of receiptconfirmation frames by the access point device AP.

For this reason, even when the access point device AP SDM-transmits dataframes to the wireless terminals STAT to STA3, the data frames can betransmitted continuously based on the concept of TXOP in IEEE802.11ewithout carrier sensing when the SIFS has passed after reception ofreceipt confirmation frames. Accordingly, MAC efficiency can beimproved.

The aforementioned synchronization of the return completion timings ofreceipt confirmation frames need not mean complete synchronization sothat the difference between the return completion timings of receiptconfirmation frames from a plurality of wireless terminals can beallowed to be not larger than the SIFS (16 microseconds).

However, for reception of receipt confirmation frames from three or morewireless terminals, it is necessary that the difference between thereturn completion timing of a receipt confirmation frame earliest incompletion of reception thereof and the return completion timing of areceipt confirmation frame latest in completion of reception thereof isnot larger than the SIFS (16 microseconds).

Although description has been made on the case where the differencebetween the return completion timings of receipt confirmation framesfrom a plurality of wireless terminals can be allowed to be not largerthan the SIFS (16 microseconds), the difference may be set, for example,to be not larger than the time length of a preamble of each data frame.

The point of difference between the return start control module 41 andthe return completion control module 42 will be described below.

The return start control module 41 synchronizes the transmissioncompletion timings of data frames transmitted to a plurality of wirelessterminals in order to synchronize the return start timings of receiptconfirmation frames returned from the plurality of wireless terminals.That is, the return start control module 41 can control data framestransmitted by the access point device AP and a transmission methodthereof without necessity of controlling the wireless terminals on thereceiver side.

On the other hand, the return completion control module 42 needs toperform control in consideration of a return method of receiptconfirmation frames returned from a plurality of wireless terminals onthe receiver side in order to synchronize the return completion timingsof the receipt confirmation frames returned from the plurality ofwireless terminals.

Items concerned with the control of the return completion control module42 will be described below.

First, in a wireless system compliant with the IEEE802.11 standard, thetype of a receipt confirmation frame to be returned when an access pointdevice or a wireless terminal receives a frame varies according to thetype of the received frame.

When, for example, an RST (Request To Send) frame which is a kind ofcontrol frame is received, the access point device or the wirelessterminal returns a CTS (Clear To Send) frame as the receipt confirmationframe.

When, for example, a data frame is received, the access point device orthe wireless terminal returns an ACK frame or a Block ACK frame as thereceipt confirmation frame or does not return any receipt confirmationframe in accordance with the Ack Policy field described in a MAC headerof the data frame.

The Block ACK frame returned as the receipt confirmation frame inresponse to reception of a data frame can be classified into a pluralityof types, that is, normal Block ACK frame, Compressed Block ACK frame(defined in IEEE802.11n) and Multi TID Block ACK frame (defined inIEEE802.11n).

For this reason, when the Ack Policy field of the data frame indicatesBlock Ack Policy, the access point device or the wireless terminalreturns any one of a normal Block ACK frame, a Compressed Block ACKframe and a Multi TID Block ACK frame as the receipt confirmation frame.

As described above, the type of a receipt confirmation frame to bereturned when an access point device or a wireless terminal receives aframe varies according to the type of the received frame.

The frame size of a receipt confirmation frame according to the type ofthe receipt confirmation frame will be described below.

In the IEEE802.11 standard, the frame size of a CTS frame or an ACKframe is 14 bytes long.

In the IEEE802.11n standard, the frame size of a normal Block ACK frameis 152 bytes long. In the IEEE802.11n standard, the frame size of aCompressed Block ACK frame is 32 bytes long. In the IEEE802.11nstandard, the frame size of a Multi TID Block ACK frame is variable and(22+10n) bytes long when n is the number of traffic identifiers (TIDs)contained in the received data frame (n is an integer not smaller than2).

As described above, the frame size of a receipt confirmation framevaries according to the type of the receipt confirmation frame.

When the access point device AP SDM-transmits different data frames(e.g. including control frames such as RST frames and Block Ack Requestframes, and management frames) to a plurality of wireless terminals, theaccess point device AP receives receipt confirmation frames different intype and frame size from the plurality of wireless terminals.

Even when the return start timings of the receipt confirmation framesreturned from the plurality of wireless terminals are synchronized withone another, the return completion timings of the receipt confirmationframes cannot be synchronized with one another in accordance with thesynchronization of the return start timings because the receiptconfirmation frames returned from the plurality of wireless terminalshave different frame sizes.

Moreover, in a wireless system compliant with the IEEE802.11 standard,various values are prepared as the transfer rate in exchange(transmission/reception) of a frame between an access point device and awireless terminal.

For this reason, even when the return start timings of receiptconfirmation frames are synchronized with one another, the returncompletion timings of the receipt confirmation frames returned to theaccess point device AP from a plurality of wireless terminals cannot besynchronized with one another in accordance with the synchronization ofthe return start timings of the receipt confirmation frames because thereceipt confirmation frames returned from the plurality of wirelessterminals are different in transfer rate as well as different in framesize according to the type of each receipt confirmation frame.

FIGS. 4A and 4B are diagrams showing examples in which the return starttimings of receipt confirmation frames returned from a plurality ofwireless terminals are synchronized with one another but the returncompletion timings of the receipt confirmation frames are notsynchronized with one another.

FIG. 4A shows the case where the transfer rates of receipt confirmationframes returned from the wireless terminals STA1 and STA2 are equally 24Mbps but the receipt confirmation frames returned from the wirelessterminals STA1 and STA2 are an Ack frame (14 bytes long) and a Block Ackframe (152 bytes long), respectively.

FIG. 4B shows the case where receipt confirmation frames returned fromthe wireless terminals STA1 and STA2 are equally of an ACK frame typebut the transfer rates of the receipt confirmation frames returned fromthe wireless terminals STA1 and STA2 are 24 Mbps and 6 Mbps,respectively.

Because receipt confirmation frames returned from a plurality ofwireless terminals (wireless terminals STA1 and STA2 in the examplesshown in FIGS. 4A and 4B are different in frame size in accordance withthe type of each receipt confirmation frame and different in transferrate in this manner, the return completion timings of the receiptconfirmation frames returned from the plurality of wireless terminalscannot be synchronized even when the return start timings thereof aresynchronized.

For this reason, even when the return start timings of receiptconfirmation frames are synchronized, there is a possibility thattransmission of data frames by the access point device AP and receptionof the receipt confirmation frames by the access point device AP willoverlap with each other to cause collision if data frames areSDM-transmitted to a plurality of wireless terminals so that the dataframes can be transmitted continuously without carrier sensing in theTXOP period.

That is, it is important to synchronize the return completion timings ofthe receipt confirmation frames returned from the plurality of ratioterminals in order to make it possible that the access point device APSDM-transmits data frames to the plurality of wireless terminals so thatthe data frames can be transmitted continuously without carrier sensingin the TXOP period.

The return completion control module 42 controls each of data framesSDM-transmitted to the plurality of wireless terminals and atransmission method thereof to thereby control each of receiptconfirmation frames returned from the wireless terminals on the receiverside and a return method thereof without awareness of the wirelessterminals on the receiver side. That is, the return completion controlmodule 42 controls each of data frames SDM-transmitted to the pluralityof wireless terminals and a transmission method thereof to therebysynchronize the reception completion timings of receipt confirmationframes returned from the wireless terminals on the receiver side.

FIG. 5 is a block diagram showing the configuration of the returncompletion control module 42.

The return completion control module 42 has a transfer rate controller43, and a type controller 44. The type controller 44 has a BA controller45. The BA controller 45 has a TID number controller 46.

For synchronization of the reception completion timings of receiptconfirmation frames returned from the wireless terminals on the receiverside, the transfer rate controller 43 of the return completion controlmodule 42 first determines the transfer rate used for SDM-transmissionof data frames to a plurality of wireless terminals. After determinationof the transfer rate, the transfer rate controller 43 of the returncompletion control module 42 notifies the return start control module 41of the determined transfer rate because the transfer rate has influenceon the time length of each data frame and is used for the return startcontrol of receipt confirmation frames.

The transfer rate controller 43 controls the transfer rate for returningreceipt confirmation frames from the plurality of wireless terminals bydetermining the transfer rate used for SDM-transmission of data framesto the plurality of wireless terminals.

In a wireless system compliant with the IEEE802.11 standard, thetransfer rate for returning a receipt confirmation frame is determinedas the highest one of transfer rates defined in a Basic Rate Set and nothigher than the transfer rate of the received data frame.

For example, assume that the Basic Rate Set is defined as {6 Mbps, 12Mbps, 24 Mbps} in the wireless system.

Example 1

When the wireless terminal STA1 receives a data frame from the accesspoint device AP at a transfer rate of 54 Mbps, the wireless terminalSTA1 returns a receipt confirmation frame at a transfer rate of 24 Mbps.

Example 2

When the wireless terminal STA1 receives a data frame from the accesspoint device AP at a transfer rate of 18 Mbps, the wireless terminalSTA1 returns a receipt confirmation frame at a transfer rate of 12 Mbps.

The transfer rate controller 43 determines transfer rates forSDM-transmission of data frames to a plurality of wireless terminals byusing the transfer rate determining mechanism at the time of returningreceipt confirmation frames in a wireless system compliant with theIEEE802.11 standard so that the transfer rates of the receiptconfiguration frames returned from the respective wireless terminals areequalized as one and the same transfer rate.

The transfer rate controller 43 determines transfer rates not lower thanthe highest one of transfer rates defined in the Basic Rate Set as thetransfer rates for SDM-transmission of data frames to the plurality ofwireless terminals.

The transfer rate controller 43 sets one of transfer rates defined inthe Basic Rate Set except the highest one of the defined transfer ratesas a first transfer rate, sets the lowest one of transfer rates definedin the Basic Rate Set and higher than the first transfer rate as asecond transfer rate, and determines transfer rates not lower than thefirst transfer rate and lower than the second transfer rate as thetransfer rates for SDM-transmission of data frames to the plurality ofwireless terminals. The transfer rate for returning receipt confirmationframes to the access point device AP from the plurality of wirelessterminals on the receiver side is equal to the first transfer rate.

When, for example, the Basic Rate Set is {6 Mbps, 12 Mbps, 24 Mbps} anda receipt conformation frame is intended to be returned from eachwireless terminal to the access point device AP at a transfer rate of 24Mbps, the transfer rate controller 43 determines the transfer rate foreach of data frames to be SDM-transmitted to the wireless terminals fromany one of 24 Mbps, 36 Mbps, 48 Mbps and 54 Mbps.

When the access point device AP transmits data frames to the wirelessterminals at the aforementioned transfer rates (each of which is any oneof 24 Mbps, 36 Mbps, 48 Mbps and 54 Mbps), the respective wirelessterminals on the receiver side return receipt confirmation frames at atransfer rate of 24 Mbps.

Accordingly, even when data frames are intended to be SD-transmitted towireless terminals so that the wireless terminals on the receiver sidecan return receipt confirmation frames at one and the same transferrate, the transfer rate controller 43 need not equalize the transferrates for SDM-transmission of data frames to the wireless terminals asone and the same transfer rate and can select and determine the transferrates independently from the aforementioned four kinds of transfer rates(24 Mbps, 36 Mbps, 48 Mbps and 54 Mbps).

Further, when, for example, the Basic Rate Set is {6 Mbps, 12 Mbps, 24Mbps} and a receipt conformation frame is intended to be returned fromeach wireless terminal to the access point device AP at a transfer rateof 6 Mbps, the transfer rate controller 43 selects and determines thetransfer rate for each of data frames to be SDM-transmitted to thewireless terminals from either of 6 Mbps and 9 Mbps.

In this manner, the transfer rate controller 43 of the return completioncontrol module 42 controls the transfer rates for data frames to beSDM-transmitted from the access point device AP to the wirelessterminals to thereby control the transfer rate for receipt confirmationframes so that the wireless terminals on the receiver side can returnthe receipt confirmation frames at one and the same transfer ratewithout awareness of the wireless terminals on the receiver side.

For synchronization of the reception completion timings of receiptconfirmation frames returned from the wireless terminals on the receiverside, the type controller 44 of the return completion control module 42determines the types of data frames to be SDM-transmitted to thewireless terminals after the transfer rate controller 43 of the returncompletion control module 42 determines the transfer rates.

The type controller 44 determines the types of data frames (includingBlock Ack Request control frames) to be SDM-transmitted to the wirelessterminals to thereby control the types of receipt confirmation frames tobe returned from the wireless terminals on the receiver side.

The type controller 44 controls the types of receipt confirmation framesto be returned from the wireless terminals on the receiver side so thatthe types of receipt confirmation frames can be equalized as one and thesame type, that is, the frame sizes of the receipt confirmation framescan be equalized as one and the same frame size. The control methodperformed by the type controller 44 will be described below.

First, the type controller 44 determines “Ack Policy” fields(hereinafter referred to as respective “Ack Policy” fields) of MACheaders of data frames to be SDM-transmitted to the wireless terminals.

The type controller 44 determines any one of “Normal Ack”, “Block Ack”and “No Ack” as a unified ACK policy for the respective “Ack Policy”fields. Data frames whose respective “Ack Policy” fields can be unifiedby the type controller 44 may be selected as the data frames to beSDM-transmitted to the wireless terminals, in advance.

Any method may be used in the type controller 44 as to which of “NormalAck”, “Block Ack” and “No Ack” is determined as a unified ACK policy forthe respective Ack Policy fields.

For example, the type controller 44 determines a unified ACK policy forthe respective “Ack Policy” fields in accordance with the traffic typesof respective data frames to be transmitted, the combination ofrespective wireless terminals on the receiver side and respective dataframes to be SDM-transmitted, the quantity of data stored in thetransmission buffer of the MAC processing module 30, etc.

In this manner, upon reception of data frames from the access pointdevice AP, the wireless terminals return receipt confirmation framesrespectively in accordance with the unified ACK policy set in the “AckPolicy” fields of the received data frames.

When “Normal Ack” is set in the “Ack Policy” field of a data frame, thewireless terminal returns an ACK frame as the receipt confirmation frameafter reception of the data frame from the access point device AP.

When “Block Ack” is set in the “Ack Policy” field of a data frame, thewireless terminal returns a Block ACK frame as the receipt confirmationframe after reception of a Block Ack Request frame following the dataframe from the access point device AP.

When “Block Ack” is set in the “Ack Policy” field of a data frame andthe data frame is A-MPDU (Aggregated-MPDU), the wireless terminalreturns a Block ACK frame as the receipt confirmation frame afterreception of the data frame (A-MPDU) from the access point device AP.

When “No Ack” is set in the “Ack Policy” field of a data frame, thewireless terminal does not return any receipt confirmation frame.

As described above, the type controller 44 determines “Block Ack” as aunified ACK policy for the respective “Ack Policy” fields, each wirelessterminal on the receiver side returns a Block ACK frame as the receiptconfirmation frame but the type of the Block ACK frame varies.

Therefore, when the type controller 44 determines “Block Ack” as aunified ACK policy for the respective “Ack Policy” fields, the BAcontroller 45 further performs control to equalize the types of BlockACK frames to be returned from the wireless terminals.

The BA controller 45 performs control to determine any one of a normalBlock ACK frame, a Compressed Block ACK frame and a Multi TID Block ACKframe as a unified type for Block ACK frames to be returned from thewireless terminals receiving data frames from the access point deviceAP.

According to the IEEE802.11n standard, when the access point device APSDM-transmits data frames (A-MPDU) to the wireless terminalsrespectively, receipt confirmation frames returned from the wirelessterminals respectively are always Compressed Block ACK frames. That is,when the BA controller 45 performs control so that data frames to beSDM-transmitted to the wireless terminals respectively are transmittedas A-MPDU frames, receipt confirmation frames to be returned from thewireless terminals respectively can be controlled to be Compressed BlockACK frames.

When the data frame is not A-MPDU, each wireless terminal returns areceipt confirmation frame to the access point device AP basically afterreception of a Block Ack Request frame. Therefore, the BA controller 45of the access point device AP controls the type of a Block ACK frame tobe returned in the category of the IEEE802.11 standard by using the MACheader of a Block Ack Request frame when the Block Ack Request frame istransmitted to each wireless terminal.

The type of the Block ACK frame to be returned can be designated by theMAC header of the Block Ack Request frame.

FIGS. 6A and 6B are diagrams showing frames transmitted/received by thewireless terminals STA1 and STA2 when the Ack Policy fields are unified.

FIG. 6A is a diagram showing transmission/reception of frames when theaccess point device AP SDM-transmits data frames (Down Data) each havingan “Ack Policy” field of “Normal Ack” to the wireless terminals STA1 andSTA2 and the types of receipt confirmation frames returned from thewireless terminals STA1 and STA2 are unified into an ACK frame.

Each of the wireless terminals STA1 and STA2 returns an ACK frame as thereceipt confirmation frame when the SIFS has passed after reception ofthe data frame (Down Data) from the access point device AP.

FIG. 6B is a diagram showing transmission/reception of frames when theaccess point device AP SDM-transmits data frames (Down Data) each havingan “Ack Policy” field of “Block Ack” and successively transmits BlockAck Requests to the wireless terminals STA1 and STA2 and the types ofreceipt confirmation frames returned from the wireless terminals STA1and STA2 are unified into a Compressed Block Ack frame.

Each of the wireless terminals STA1 and STA2 returns a Compressed BlockAck frame as the receipt confirmation frame when the SIFS has passedafter reception of the Block Ack Request from the access point deviceAP.

Any method may be used as the control method performed by the BAcontroller 45 if the types of Block Ack frames returned from a pluralityof wireless terminals can be equalized.

A Compressed Block ACK frame and a Multi TID Block frame are framesnewly defined in the IEEE802.11n standard. For this reason, when anywireless terminal not compliant with the IEEE802.11n standard iscontained in the wireless terminals on the receiver side ofSDM-transmission of data frames from the access point device AP, the BAcontroller 45 unifies the types of the Block Ack frames returned fromthe wireless terminals, into a normal Block ACK frame.

The Compressed Block ACK frame is a receipt confirmation framecorresponding to a data frame not based on such fragmentation that thedata frame is fragmented into parts. For this reason, when the BAcontroller 45 performs control so that the types of Block ACK frames tobe returned from the wireless terminals are unified into a CompressedBlock ACK frame, each of data frames to be SDM-transmitted from theaccess point device AP to the wireless terminals is not fragmented intoparts.

Description will be made on the case where the BA controller 45 unifiesthe types of receipt confirmation frames to be returned from thewireless terminals into a Multi TID Block Ack frame when the accesspoint device AP SDM-transmits data frames each having an “Ack Policy”field of “Block Ack” to the wireless terminals.

When the BA controller 45 unifies the types of receipt confirmationframes to be returned from the wireless terminals into a Multi TID BlockAck frame, the TID number controller 46 further performs control toequalize the numbers of TIDs in Multi TID Block Ack frames to bereturned from the wireless terminals respectively.

Control of the number of TIDs can be designated in the same manner asthe type of a Block ACK frame by controlling each of data frames to beSDM-transmitted in the category of the IEEE802.11 wireless LAN standardsuch as Block Ack Request.

As described above, when the access point device AP SDM-transmits dataframes to the wireless terminals, not only the transmission completiontimings of the data frames to the wireless terminals can be synchronizedbut also the return completion timings of receipt confirmation framesfrom the wireless terminals can be synchronized without any specialprocess request on the respective wireless terminals, but by controllingthe type of each of the data frames to be transmitted to the wirelessterminals, the transfer rate for transmission of each data frame, the“Ack Policy” field of the MAC header of each data frame, the type ofeach Block Ack frame requested by Block Ack Request, the number of TIDsin each Multi TID Block ACK frame, etc.

In this manner, in the wireless system according to the firstembodiment, when the access point device AP SDM-transmits data frames tothe wireless terminals, the reception completion timings of receiptconfirmation frames returned from the wireless terminals can besynchronized. Accordingly, even when the access point device AP performscontinuous transmission of data frames by using the concept of TXOPdefined in the IEEE802.11e standard while transmitting data frames tothe wireless terminals simultaneously on one and the same frequencychannel by SDM, collision can be prevented from being caused by overlapbetween the transmission timings of data frames by the access pointdevice AP and the reception timings of receipt confirmation frames bythe access point device AP. Accordingly, the access point device APaccording to the first embodiment can transmit data frames continuouslywithout carrier sensing by using the concept of TXOP whileSDM-transmitting data frames to the wireless terminals, so thatimprovement of MAC efficiency can be achieved.

In the wireless system according to the first embodiment, though theaccess point device AP transmits data frames simultaneously and on oneand the same frequency channel, the reception completion timings ofreceipt confirmation frames returned from the wireless terminals can besynchronized by a simple method in which each wireless terminaltransmits a receipt confirmation frame when the SIFS has passed afterreception of a corresponding one of the data frames SDM-transmitted fromthe access point device AP, in accordance with the IEEE802.11 (includingIEEE802.11e and IEEE802.11n) standard.

In the wireless system according to the first embodiment, when theaccess point device AP SDM-transmits data frames to the wirelessterminals, each of the data frames and a transmission method thereof arecontrolled but each wireless terminal need not be notified of anyspecial information. Accordingly, it is unnecessary to add any newprocessing module to each wireless terminal.

In the wireless system according to the first embodiment, the accesspoint device AP performs control by using the IEEE802.11 standardeffectively and transmits data frames to the wireless terminals inaccordance with the control. Accordingly, each wireless terminal canreturn a desired receipt confirmation frame without any awareness, sothat the reception completion timings of the receipt confirmation framesreturned from the wireless terminals can be synchronized.

The first embodiment has been described on the assumption that SDM isused when the access point device AP transmits (multiplexes andtransmits) data frames to the plurality of wireless terminalssimultaneously, and that data frames are transmitted simultaneously andon one and the same frequency channel.

However, in the first embodiment, another multiplexing method such asFDM (Frequency Division Multiplexing) or CDM (Code DivisionMultiplexing) than SDM may be used when the access point device APmultiplexes and transmits data frames to the plurality of wirelessterminals.

In this manner, the access point device AP can transmit data framescontinuously based on the concept of TXOP without carrier sensing whilethe access point device AP multiplexes and transmits data frames to theplurality of wireless terminals, so that improvement of MAC efficiencycan be achieved.

The access point device AP can be achieved, for example, by ageneral-purpose computer used as basic hardware. That is, the MACprocessing module 30, the controller 40 (including the return startcontrol module 41, the return completion control module 42, the transferrate controller 43, the type controller 44, the BA controller 45, andthe TID number controller 46) and the multiplexing module 50 can beachieved by a program executed by a processor mounted in the computer.The access point device AP may be achieved by the program installed inthe computer in advance or by the program stored in a recording mediumsuch as a CD-ROM or distributed through a network and suitably installedin the computer. The transmission buffer (not shown) of the MACprocessing module 30 can be achieved by suitable use of a built-in orexternal memory of the computer, a hard disk or a recording medium suchas a CD-R, a CD-RW, a DVD-RAM or a DVD-R.

Second Embodiment

In the wireless system according to the first embodiment, when theaccess point device AP SDM-transmits data frames to the plurality ofwireless terminals, the access point device AP controls each data frameand a transmission method thereof so that the reception completiontimings of receipt confirmation frames returned from the wirelessterminals can be synchronized with one another without the wirelessterminals' awareness.

In a wireless system according to a second embodiment, the access pointdevice AP designates a returning method of each receipt confirmationframe explicitly to the plurality of wireless terminals so that thereception completion timings of receipt confirmation frames returnedfrom the wireless terminals can be synchronized with one another.

In the wireless system according to the second embodiment, because thereturning method of each receipt confirmation frame is designatedexplicitly, the access point device AP can determine each data frame anda transmission method thereof freely without any restriction when theaccess point device AP SDM-transmits data frames to the plurality ofwireless terminals.

FIG. 7 is a block diagram showing an example of configuration of theaccess point device AP according to the second embodiment.

The configuration of the access point device AP according to the secondembodiment is different from that of the access point device APaccording to the first embodiment in the operation of the controller140. The controller 140 according to the second embodiment has the samefunction as the return start control module 41 according to the firstembodiment.

When or before the access point device AP SDM-transmits data frames to aplurality of wireless terminals, the controller 140 determines areturning method of a receipt confirmation frame from each wirelessterminal so that the return completion timings of receipt confirmationframes returned from the plurality of wireless terminals can besynchronized with one another. The controller 140 determines the receiptconfirmation frame returning method such as a transfer rate, an AckPolicy, etc. in accordance with each of data frames to beSDM-transmitted by the access point device AP.

The access point device AP notifies each wireless terminal of thereceipt confirmation frame returning method determined by the controller140 so that the return completion timings of receipt confirmation framesreturned from the wireless terminals can be synchronized with oneanother.

The access point device AP may notify each wireless terminal of thereceipt confirmation frame returning method when the access point deviceAP SDM-transmits data frames to the plurality of wireless terminals.Alternatively, the access point device AP may notify each wirelessterminal of the receipt confirmation frame returning method (at the timeof Association, etc.) before the access point device AP SDM-transmitsdata frames to the plurality of wireless terminals. Alternatively, theaccess point device AP may notify each wireless terminal of the receiptconfirmation frame returning method periodically.

In the wireless system according to the second embodiment, the accesspoint device AP notifies each wireless terminal of the receiptconfirmation frame returning method explicitly when the access pointdevice AP SDM-transmits data frames to a plurality of wirelessterminals. Each wireless terminal returns a receipt confirmation framein accordance with the returning method given from the access pointdevice AP. The access point device AP and each wireless terminal operatein this manner to synchronize the reception completion timings ofreceipt confirmation frames with one another.

Three examples of a method by which the access point device AP notifieseach wireless terminal of the “receipt confirmation frame returningmethod” explicitly will be described below.

In a first notification method, the access point device AP writes the“receipt confirmation frame returning method” in the MAC header field ofeach of data frames to be transmitted and then SDM-transmits the dataframes whenever the access point device AP SDM-transmits data frames toa plurality of wireless terminals. The field of the MAC header of eachdata frame in which the “receipt conformation frame returning method” iswritten is not defined in the current IEEE802.11 standard. The fieldwill be defined in a standard designed in the future.

In a second notification method, the access point device AP sends noticeof the “receipt confirmation frame returning method” to wirelessterminals connected to one and the same network at the time of startingthe network or during establishment of the network. The access pointdevice AP writes the “receipt confirmation frame returning method” in aBroadcast frame (a report frame such as a Beacon frame or a ProbeResponse frame) and transmits the Broadcast frame to all the wirelessterminals connected to one and the same network.

In a third notification method, the access point device AP sends advancenotice of the “receipt confirmation frame returning method” to wirelessterminals connected to one and the same network at the time of startingthe network or during establishment of the network by using a Multicastframe in accordance with each group of wireless terminals.

As described in the second notification method, when advance notice ofthe “receipt confirmation frame returning method” is sent by use of aBroadcast frame, the access point device AP can designate one and thesame “receipt confirmation frame returning method” to all the wirelessterminals connected to the network.

As described in the third notification method, when advance notice ofthe “receipt confirmation frame returning method” is sent by use of aMulticast frame, the access point device AP can designate a “receiptconfirmation frame returning method” in accordance with each group ofwireless terminals (hereinafter referred to as SDM-grouping) to whichdata frames can be transmitted at once.

The third notification method (advance notice using a Multicast frame)can perform ad hoc control because the access point device AP candesignate the receipt confirmation frame returning method in accordancewith SDM-grouping. In the third notification method, the access pointdevice AP needs to send notice of the “receipt confirmation framereturning method” whenever SDM-grouping is changed.

The configuration of the access point device AP shown in FIG. 7 is anexample of configuration in the case where the access point device APsends notice of the “receipt confirmation frame returning method” toeach wireless terminal by the first notification method (the second orthird notification method may be further used).

In the configuration of the access point device AP in the case where theaccess point device AP sends notice of the “receipt confirmation framereturning method” to each wireless terminal not by the firstnotification method but by the second or third notification method, itis unnecessary to connect the controller 140 to the multiplexing module50 directly. For example, the access point device AP may be configuredas shown in FIG. 8.

The operation of the controller 140 will be described below in the casewhere the access point device AP sends notice of the “receiptconfirmation frame returning method” to each wireless terminal so thatthe return completion timings of receipt confirmation frames returnedfrom the wireless terminals can be synchronized with one another.

The controller 140 determines the type of each receipt confirmationframe and the transfer rate in returning of the receipt confirmationframe. The type of each receipt confirmation frame and the transfer ratein returning of the receipt confirmation frame have been described inthe first embodiment.

For example, in the first notification method, the controller 140 writes{receipt confirmation frame=ACK frame, transfer rate of receiptconfirmation frame=24 Mbps} in the MAC header field of each of dataframes to be SDM-transmitted to a plurality of wireless terminals by theaccess point device AP.

For example, in the second or third notification method, the controller140 writes {receipt confirmation frame=Block ACK frame, transfer rate ofreceipt confirmation frame=9 Mbps} in a Broadcast frame or a Multicastframe to be transmitted by the access point device AP.

In this manner, the controller 140 sends of notice of the type of areceipt confirmation frame returned from each wireless terminal and thetransfer rate in returning of the receipt confirmation frame, explicitlyin accordance with each wireless terminal or in accordance with eachSDM-grouping or in accordance with each network and performs control. Asa result, when the access point device AP SDM-transmits data frames tothe wireless terminals respectively, the type of each data frame and thetransfer rate in transmission of the data frame can be selected and setindependently.

In this manner, in the wireless system according to the secondembodiment, the access point device AP sends notice of the receiptconformation frame returning method to each wireless terminalexplicitly. As a result, when the access point device AP SDM-transmitsdata frames to a plurality of wireless terminals, the receptioncompletion timings of receipt confirmation frames returned from theplurality of wireless terminals can be synchronized with one another.

For this reason, even when the access point device AP transmits dataframes continuously based on the concept of TXOP defined in theIEEE802.11e standard while transmitting data frames to a plurality ofwireless terminals simultaneously and on one and the same frequencychannel by SDM, collision can be prevented from being caused by overlapbetween the transmission timings of data frames by the access pointdevice AP and the reception timings of receipt confirmation frames bythe access point device AP.

Accordingly, the access point device AP according to the secondembodiment can transmit data frames continuously based on the concept ofTXOP without carrier sensing while SDM-transmitting data frames to aplurality of wireless terminals, so that improvement of MAC efficiencycan be achieved.

In the IEEE802.11n standard, each access point device AP and eachwireless terminal are allowed to add a 32-byte field called “HT Controlfield” to a frame regardless of the type of the frame (inclusive of areceipt confirmation frame).

That is, in the IEEE802.11n standard, each wireless terminal is allowedto determine arbitrarily whether or not the HT Control field is to beadded to a receipt confirmation frame to be returned to the access pointdevice AP.

For this reason, even when the access point device AP designates thetype of a receipt confirmation frame to be returned from each wirelessterminal and the transfer rate in returning of the receipt confirmationframe, there is a possibility that the return completion timings ofreceipt confirmation frames returned from the wireless terminals willnot be synchronized with one another.

Therefore, the controller 140 of the access point device AP according tothe second embodiment may send notice of forbidden addition of the HTControl field or always addition of the HT Control field to a receiptconfirmation frame in addition to the type of the receipt confirmationframe and the transfer rate in returning of the receipt confirmationframe when the network is compliant with the IEEE802.11n standard.

In this manner, even when the wireless system is compliant with theIEEE802.11n standard and each wireless terminal is allowed to add the HTControl field to a receipt confirmation frame arbitrarily, it ispossible to synchronize the return completion timings of receiptconfirmation frames returned from the wireless terminals to the accesspoint device AP.

The controller 140 according to the second embodiment has the samefunction as the return start control module 41 according to the firstembodiment.

However, the controller 140 according to the second embodiment may sendnotice of the return start timing of a receipt confirmation frameexplicitly to each wireless terminal in place of the same function asthe return start control module 41 according to the first embodiment inaddition to notice of the type of the receipt confirmation frame to bereturned from each wireless terminal, the transfer rate in returning ofthe receipt confirmation frame and information as to whether not onlythe HT Control field is to be added.

Further, the controller 140 according to the second embodiment sendsnotice of the type of the receipt confirmation frame to be returned fromeach wireless terminal, the transfer rate in returning of the receiptconfirmation frame and information as to whether not only the HT Controlfield is to be added, explicitly to each wireless terminal, so that thereturn completion timings of receipt confirmation frames from thewireless terminals can be synchronized with one another.

However, the controller 140 according to the second embodiment may sendnotice of the return completion timing of a receipt confirmation frameexplicitly to each wireless terminal in place of explicit notice of theaforementioned information.

Each wireless terminal explicitly notified of the return completiontiming of the receipt confirmation frame performs control so that thereturn completion timing of the receipt completion frame actuallyreturned from the wireless terminal is equalized to the returncompletion timing of the receipt confirmation frame given from theaccess point device AP.

In this manner, even when the access point device AP SDM-transmits dataframes to a plurality of wireless terminals, it is possible tosynchronize the return completion timings of receipt confirmation framesreturned from the wireless terminals with one another.

If the return completion timing of the receipt confirmation framereturned from each wireless terminal can be synchronized with the returncompletion timing of the receipt confirmation frame given from theaccess point device AP, each wireless terminal may return any type ofreceipt confirmation frame and may return the receipt confirmation frameby any returning method (such as the transfer rate and the return starttiming of the receipt confirmation frame).

The wireless system according to the second embodiment may use anothermultiplexing method such as FDM (Frequency Division Multiplexing) or CDM(Code Division Multiplexing) than SDM when the access point device APmultiplexes and transmits data frames to a plurality of wirelessterminals, in the same manner as the wireless system according to thefirst embodiment.

It is to be understood that the present invention is not limited to thespecific embodiments described above and that the present invention canbe embodied with the components modified without departing from thespirit and scope of the present invention. The present invention can beembodied in various forms according to appropriate combinations of thecomponents disclosed in the embodiments described above. For example,some components may be deleted from the configurations described as theembodiments. Further, the components described in different embodimentsmay be used appropriately in combination.

What is claimed is:
 1. A wireless apparatus comprising: a receiverconfigured to receive a first frame among a plurality of framesincluding the first frame and a second frame that is to be transmittedby multiplexing from a wireless communication apparatus, a transmitterconfigured to transmit a third frame, wherein a length of the thirdframe varies according to at least first content of the first frame, atemporal difference between a first time at which the third frame isfully received by the wireless communication apparatus and a second timeat which a fourth frame sent in response to the second frame is fullyreceived by the wireless communication apparatus is less than apredetermined time length.
 2. The apparatus of claim 1, wherein the atleast first content of the first frame is information for setting atransmission rate of the third frame.
 3. The apparatus of claim 1,wherein the at least first content of the first frame is information forsetting a frame type of the third frame.
 4. The apparatus of claim 1,wherein the at least first content of the first frame is Ack policy forthe third frame.
 5. The apparatus of claim 1, wherein the at least firstcontent of the first frame is information for setting a number of TIDsof the third frame.
 6. The apparatus of claim 1, wherein the at leastfirst content of the first frame is information included in a header ofthe first frame.
 7. The apparatus of claim 1, wherein the temporaldifference is zero.
 8. The apparatus of claim 1, further comprising: atleast one antenna for transmitting a radio wave.
 9. A wireless apparatuscomprising: a receiver configured to receive a first frame among aplurality of frames including the first frame and a second frame that isto be transmitted by multiplexing from a wireless communicationapparatus, a transmitter configured to transmit a third frame, whereinthe receiver is configured to receive notification information from thewireless communication apparatus, a length of the third frame variesaccording to the notification information, a temporal difference betweena first time at which the third frame is fully received by the wirelesscommunication apparatus and a second time at which a fourth frame sentin response to the second frame is fully received by the wirelesscommunication apparatus is less than a predetermined time length. 10.The apparatus of claim 9, wherein the notification information isinformation for setting a transmission rate of the third frame.
 11. Theapparatus of claim 9, wherein the temporal difference is zero.
 12. Theapparatus of claim 9, further comprising: at least one antenna fortransmitting a radio wave.
 13. A wireless apparatus comprising: areceiver configured to receive, from a wireless communication apparatus,a first frame including first information for the wireless apparatus andsecond information for another wireless apparatus, a transmitterconfigured to transmit a second frame to the wireless communicationapparatus, wherein a length of the second frame varies according to atleast the first information, a third frame is transmitted to thewireless communication apparatus from the another wireless apparatus,and a temporal difference between a first receipt completion time atwhich the second frame is fully received by the wireless communicationapparatus and a second receipt completion time at which the third frameis fully received by the wireless communication apparatus is less than apredetermined time length.
 14. The apparatus of claim 13, wherein thefirst information is information for setting a transmission rate of thethird frame.
 15. The apparatus of claim 13, wherein the temporaldifference is zero.
 16. The apparatus of claim 13, further comprising:at least one antenna for transmitting a radio wave.
 17. A wirelessmethod comprising: receiving a first frame among a plurality of framesincluding the first frame and a second frame that is to be transmittedby multiplexing from a wireless communication apparatus, transmitting athird frame, wherein a length of the third frame varies according to atleast first content of the first frame, a temporal difference between afirst time at which the third frame is fully received by the wirelesscommunication apparatus and a second time at which a fourth frame sentin response to the second frame is fully received by the wirelesscommunication apparatus is less than a predetermined time length. 18.The method of claim 17, wherein the at least first content of the firstframe is information for setting transmission rate of the third frame.19. The method of claim 17, wherein the temporal difference is zero. 20.The method of claim 17, wherein the at least first content of the firstframe is information included in a header of the first frame.